1. Field of the Invention
This invention relates to totally or partially synthetic bearing greases containing thickeners and antioxidants among other additives for sealed-for-life application or long turnaround time lubrication under high temperature, high speed and severely oxidative conditions.
2. Description of the Related Art
Long term lubrication of modern bearings requires greases which stand up well to high temperatures and oxidizing environments. Lubricant performance is especially severely tested in sealed for life applications, where lubricant replacement is not contemplated and lubricant wear or breakdown will not be appreciated until after bearing failure.
The lubricating industry has turned to synthetic base stocks such as polyalpha olefins and polyol ester base stocks for grease formulations.
U.S. Pat. No. 4,406,800 discloses a grease useful over a wide temperature range comprising a polyalpha olefin base fluid and a thickener.
U.S. Pat. No. 4,075,112 and U.S. Pat. No. 4,075,113 disclose grease compositions comprising 2-15 wt % aluminum soap, 25 to 97 wt % of a hydrogenated or non-hydrogenated polymer of a monoolefin hydrocarbon having 4 carbon atoms and a mean molecular weight of between 300 and 2500, and 0-60 wt % of a lubricating oil which is preferably a mineral oil.
Japanese Patent Publication 55-9109595 discloses a lithium soap which may include a refined base stock derived from naphthenic, paraffinic and mixed base crudes. Synthetic lubricating oils are also disclosed as being useful. These include polymers and copolymers of alpha-olefins.
U.S. Pat. No. 4,749,502 discloses a grease composition comprising an oil component having a major amount of a synthetic fluid having a viscosity of at least 50 cSt at 40.degree. C. and a minor amount of a mineral oil having a pour point below -20.degree. C. and a thickener. The synthetic fluid component is selected from polyolefins, esters, diesters, polyol esters, polyesters, high VI isoparaffins and mixtures thereof, preferably polyalpha-olefins. The high VI isoparaffins are described as those materials produced by hydrocracking or hydroisomerization of waxes. The thickeners are typically simple calcium, lithium, aluminum and/or barium soap of a fatty acid, such as stearic or 12-hydroxy stearic acid or the complex calcium, lithium, barium and/or aluminum soaps/salts of the fatty acids with lower molecular weight mono or diacid (e.g., azelaic) or benzoic acid, or a modified clay thickener. The greases also contain an additive package comprising an extreme pressure additive, an anti-wear additive, an anti-rust additive and an antioxidant. Preferred antioxidants are PANA, alkyl substituted aromatic amines, etc.
U.S. Pat. No. 5,059,299 and U.S. Pat. No. 5,158,671 teach a method for isomerizing waxes to lube base oils and a method for stabilizing said hydroisomerates, respectively. The patent recites that the hydroisomerates made are usable as lube base stocks and blending stocks. The isomerates are made by hydroisomerizing slack waxes secured by the solvent dewaxing of lubricating oil boiling range base stocks, e.g. 600N oil.
It would be desirable if a grease capable of performing for extended period of time at high temperatures/speeds and in an oxidizing environment could be formulated to exceed the performance of mineral oil-based greases yet at lower cost than polyalpha olefins or polyol ester-based greases.